Reproducing the Issue

Enter the account and all respective details into the respective fields.

Choose the View All button.

Select the Items tab and add the respective project service.

Select the line item and add one project

Now when you try to add the next item the system is not displaying any projects.

The system displays project tasks only.

Cause

It is not possible and also currently not planned to be implemented in a future release. The issue is if you assign multiple projects to one sales order and assign these projects to multiple sales orders, you can build a net of projects/sales orders for which the system would not be able to perform revenue recognition and will not be able to determine profitability correctly.